二维不可压流体Kelvin-Helmholtz不稳定性的弱非线性研究

通过将扰动速度势展至三阶,提出了Kelvin-Helmholtz（KH）不稳定性的弱非线性理论.在模耦合过程中观察到一个重要的共振现象,共振使得模耦合过程变得相当复杂,单模扰动很快进入非线性区,产生大量高次谐波,共振加强了非线性作用.分析了单模扰动中二次和三次谐波产生效应,以及对基模指数增长的非线性校正.模拟结果支持了解析理论.利用该理论,分析了KH不稳定的非线性阈值问题. 关键词： Kelvin-Helmholtz不稳定性 弱非线性理论 非线性阈值     

基于色散不对称光纤环形镜的锁模光纤激光器

在理论上数值计算了各种常量对色散不对称非线性光纤环形镜透射特性的影响。并分析了它作为被动锁模器件用于光纤激光器压窄脉冲的物理机制。利用8字形主被动混合锁模的结构在调制频率为9．998748700 GHz，波长为1566．65nm处获得了11ps的稳定锁模脉冲输出．对应谱宽0．297nm．同时在重复频率为9．995792825 GHz和9．996778256 GHz时分别得到了振幅均匀的二阶和三阶谐波锁模输出。证明色散不对称非线性光纤环形镜可以有效消减脉冲的尾翼和噪声．得到脉幅稳定的脉冲序列。     

毫米波TE10-TE11-HE11模式变换器的研究

设计了一种中心频率为105GHz、带宽为10GHz的TE₁₀-TE₁₁-HE₁₁模式变换器,用于电子回旋共振波加热系统(ECRH)低功率测量平台的搭建,其中采用矩-圆过渡及渐变槽深波纹喇叭实现相关模式转换。仿真结果表明,频率范围为92～115GHz时,该模式转换器产生HE11模的转换效率即高斯含量达到95%以上。     

EAST等离子体电子温度涨落特征的初步实验研究

在EAST上使用相关电子回旋辐射(CECE)诊断系统观测到不同等离子体参数下的电子温度涨落特征,介绍了欧姆放电、L模放电及无ELM的H模放电的三种现象。在欧姆密度爬升等离子体中,电子温度涨落与电子密度之间表现出很强的相关性,即存在电子温度涨落处于较高水平的电子密度的窗口。初步分析表明,电子温度涨落变化是电子密度梯度和电子温度梯度共同影响的结果。不同辅助加热下的L模等离子体中,电子温度涨落的频谱表现出不同的行为。由于电子回旋共振加热(ECRH)的功率有限,其对电子温度的改变很小,而中性束注入(NBI)有较高的注入功率,能够明显提升电子温度,加热方式及加热功率大小引起的电子温度变化与电子温度涨落变化相关。在没有边缘局域模(ELM)的H模期间,可以观测到频率为18kHz的准相干模,其存在于归一化半径ρ=0.71～0.87较宽的径向范围内。     

8字形主被动锁模掺Er3+光纤激光器

在理论上分析了利用非线性光学环形镜作为等效可饱和吸收体压缩脉冲进行锁模的物理机制。利用8字形主被动混合锁模的结构在调制频率2．498749GHz下，在1．543μm处获得了12ps的锁模脉冲输出，对应谱宽0．22nm，时间脉宽积0．33。在抽运功率50mW情况下，输出脉冲平均功率3．715mW。在调制频率2．499344GHz、2．499114GHz和2．498999GHz时分别并获得了2～4阶幅度较为均衡的有理数谐波锁模脉冲序列。     

红外激光场诱发下的多原子分子离解反应模型

本文在以前工作的基础上,继续讨论了多原子分子在红外激光场作用下产生多光子离解的机制,得到了分子各个振动模之间的非线性耦合方程组,以SF₆分子为例,数值计算了模-模间的能量转移时间、费密共振作用、各个模被激光场激发的程度,最后联系到单分子反应的模型,讨论了多原子分子的离解速率,基本上得到了与实验现象一致的结果。 关键词：     

脉冲幅度均匀化的有理谐波锁模光纤激光器

提出并实现了一种脉冲幅度可均匀化的有理谐波锁模光纤激光器。该激光器采用主动有理谐波锁模机制,可获得重复频率为整数倍调制频率的锁模脉冲信号。通过在光纤激光器谐振腔中添加非线性光纤放大环镜,并利用其反射率对输入信号强度的开关特性,实现对锁模脉冲信号的整幅。在1 GHz的调制频率下,分别获得了4 GHz和5 GHz重复频率的锁模脉冲信号输出,并且在一定的980 nm抽运功率下,可通过调节电光调制器的直流偏置电压大小以及调制信号的调制深度,使得脉冲幅度具有较好的均匀性。     

用电子迴旋共振加热抑制托卡马克中的撕裂模

利用电子迴旋共振加热的定域性,改善等离子体电流分布,从而改善撕裂模的稳定条件,只要电子迴旋共振加热的吸收区能覆盖住撕裂模的共振层,就有很好的稳定效应;而不一定要求电子迴旋共振面与撕裂模的共振面重合,在迴旋加热期间,撕裂模被有效地抑制住。 关键词：     

HL-1M装置ECRH系统的研制与实验

在HL-1M装置上研制了一套75GHz、500kW／50ms的大型电子回旋共振加热系统，设计了准光学传输系统和顶部、水平两套天线，对系统进行了调试，获得了最佳的系统参数和运行条件，并开展了各种O模和X模实验。在单管250kW的基波0模的实验中，获得电子温度提高40％，电流位形负剪切分布以及鱼骨模不稳定性研究等重要的物理实验结果。     

有理数谐波主被动锁模掺铒光纤激光器

在理论上详细分析了利用非线性光学环形镜（NOLM）来减小输出脉冲幅度波动,消除噪声并对脉冲进行压缩整形的物理机制。在主动锁模掺铒光纤环形激光器中（AHML－EDFL）接入一个非线性光学环形镜,形成结构新颖的主被动锁模掺铒光纤激光器（APHML－EDFL）,利用非线性光学环形镜所具有的饱和吸收体功能,成功地制抑了4阶有理数谐波锁模（RHML）中较大的幅度噪声,在1GHz量级的调制频率下,由主被动锁模掺铒光纤激光器获产生重复频率为5．1GHz,幅度相当稳定的4阶有理数谐波锁模脉冲序列。     

相干彩虹的形成机制

用一束白光激光聚焦照射液体(水、丙酮、无水乙醇、汽水等)或固体(冰、有色玻璃、塑料、彩色蜡等),出现了多级的彩色干涉环,即相干彩虹.高强度白光局部地加热了液体(固体),改变了它的密度(以及折射性质),从而导致光程差的出现,不同波长的光都发生干渉,形成了彩色的干涉环.有色玻璃在反射模式下也出现了相干彩虹,此时的干涉完全来自于玻璃表面轮廓的变化,并且无参数拟合的结果定量地符合观测到的干涉图案.     

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基于托卡马克实验的L-H模转换过程中等离子体的输运特征,建立了一种简化的非线性等离子体输运模型。通过数值求解输运方程,得到了等离子体稳态温度分布剖面,成功模拟了托卡马克边缘输运势垒(ETB)的形成以及高辅助加热功率下的内部输运势垒(ITB)现象,并对比了有无输运势垒两种情况下托卡马克的能量约束效率。     

Operational Characteristics of a 14-W 140-GHz Gyrotron for Dynamic Nuclear Polarization

The operating characteristics of a 140-GHz 14-W long pulse gyrotron are presented. The device is being used in dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP/NMR) spectroscopy experiments. The gyrotron yields 14 W peak power at 139.65 GHz from the TE(0,3) operating mode using a 12.3-kV 25-mA electron beam. Additionally, up to 12 W peak has been observed in the TE(2,3) mode at 136.90 GHz. A series of mode converters transform the TE(0,3) operating mode to the TE(1,1) mode. Experimental results are compared with nonlinear simulations and show reasonable agreement. The millimeter-wave output beam was imaged in a single shot using a pyroelectric camera. The mode patterns matched reasonably well to theory for both the TE(0,1) mode and the TE(1,1) mode. Repeatable mode patterns were obtained at intervals ranging from 0.8 s apart to 11 min apart at the output of the final mode converter.     

Effect of nonlinear gain on the phase noise of Y-branch lasers

Effect of nonlinear gain on the phase noise of modulated grating Y-branch (MGY) lasers is investigated by experiments and simulations. The phase noise is first characterized by measuring the frequency modulation noise spectrum of the MGY laser, and some interesting phenomena are observed. In order to understand the underlying physic mechanism of those phenomena, simulations are performed with taking Langevin noise sources and nonlinear gain terms into account. Simulated results show that, in the presence of the nonlinear gain, fluctuations of side-modes can bring excess phase noise to the lasing mode, especially when the side-modes are on the long-wavelength side of the main mode. Furthermore, it has been found that the phase noise hopping phenomenon can be induced by a bi-stable state of the laser, which is also closely related to the nonlinear gain. The investigation is helpful for explaining the complicated phase noise characteristics of the MGY laser.     

Viscous heating and the stability of newtonian and viscoelastic taylor-couette flows

The effects of viscous heating on the stability of Taylor-Couette flow were investigated through flow visualization experiments for Newtonian and viscoelastic fluids. For highly viscous Newtonian fluids, viscous heating drives a transition to a new, oscillatory mode of instability at a critical Reynolds number significantly below that at which the inertial transition is observed in isothermal flows. The effects of viscous heating may explain the discrepancies between the observed and predicted critical conditions and the symmetry of the disturbance flow for viscoelastic instabilities.     

A 0.33-THz second-harmonic frequency-tunable gyrotron

Dynamics of the axial mode transition process in a 0.33-THz second-harmonic gyrotron is investigated to reveal the physical mechanism of realizing broadband frequency tuning in an open cavity circuit. A new interaction mechanism about propagating waves, featured by wave competition and wave cooperation, is presented and provides a new insight into the beam-wave interaction. The two different features revealed in the two different operation regions of low-order axial modes(LOAMs) and high-order axial modes(HOAMs) respectively determine the characteristic of the overall performance of the device essentially. The device performance is obtained by the simulation based on the time-domain nonlinear theory and shows that using a 12-kV/150_(-mA) electron beam and TE_(-3,4) mode, the second harmonic gyrotron can generate terahertz radiations with frequency-tuning ranges of about 0.85 GHz and 0.60 GHz via magnetic field and beam voltage tuning,respectively. Additionally, some non-stationary phenomena in the mode startup process are also analyzed. The investigation in this paper presents guidance for future developing high-performance frequency-tunable gyrotrons toward terahertz applications.     

A theoretical estimation of the pre-breakdown-heating time in the underwater discharge acoustic source

One of the common characteristics of the electrothermal breakdown in an underwater discharge acoustic source(UDAS) is the existence of a pre-breakdown-heating phase.In our experiment,two phenomena were observed:(1) the breakdown time that takes on high randomicity and obeys a "double-peak" stochastic distribution;(2) the higher salt concentration that reduces the residual voltage and causes 100% non-breakdown.The mechanism of electrothermal breakdown is analysed.To specify the end of the pre-breakdown-heating phase,a "border boiling" assumption is proposed,in which the breakdown time is assumed to be the time needed to heat the border water around the initial arc to 773 K.Based on this ’border boiling’ assumption,the numerical simulation is performed to evaluate the effects of two heating mechanisms:the Joule heating from the ionic current,and the radiation heating from the initial arc.The simulation results verify the theoretical explanations to these two experiment phenomena:(1) the stochastic distribution of the radius of the initial arc results in the randomicity of the breakdown time;(2) the difference in efficiency between the radiation heating and the Joule heating determines that,in the case of higher salt concentration,more energy will be consumed in the pre-breakdown-heating phase.     

基于聚合经验模态分解方法的OJ 287 射电流量变化周期分析

BL Lac 天体OJ 287 是一个重要的Blazar天体, 光变具有准周期特点. 本文从密歇根大学射电天文台数据库收集了OJ 287射电波段4.8, 8.0和 14.5 GHz超过30年时间的观测数据. 由于天文观测资料的复杂性, 一些传统寻找周期的方法不是太理想. 近几年迅速发展起来的聚合经验模态分解(EEMD) 特别适合于具有非线性和非平稳动态变化特性的Blazar天体光变规律的研究. 运用EEMD方法对OJ 287 的3个射电波段流量分别进行多时间尺度分解, 各获得6个代表不同时间尺度局部特征信息的本征模态函数分量和一个趋势项. 研究结果表明: 4.8 GHz射电流量变化的主要周期是18.9, 11.9, 5.7 和2.4年; 8.0 GHz 的主要周期是12.2, 5.2和2.4 年; 14.5 GHz的主要周期是21.8, 12.0, 4.3和 2.4 年. 尽管3个波段流量变化主要周期大小有一定的差别, 但是存在着近似相同的变化周期12.0 和2.4年, 说明这3个波段可能来源于相同的辐射机制, 也证实了OJ 287 射电波段和光学波段具有相同的长光变周期12.0 年. 这些结论表明: EEMD 作为一种全新的非线性、非平稳信号处理方法, 可以更精确地提取Blazar天体中不同光变周期的分量. 关键词： OJ 287 光变周期 聚合经验模态分解     

Liquid–liquid structure transition in Sn–3.5Ag–3.5Bi melts

The liquid–liquid structure transition (LLST) as the function of temperature and time in Sn–3.5Ag–3.5Bi melts was investigated with the help of direct current four-probe method. The LLST which occurs during first cycle heating of two cycles heating/cooling experiments can divide into two different structure changes: irreversible LLST of 650°C isothermal and step reversible LLST at 770°C–806°C on subsequent heating process. Obvious kinetic phenomena are observed during isothermal experiments. Irreversible and reversible LLST are analysed from the viewpoint of short-range order. These results will help to understand the law and mechanism of liquid field, and provide some scientific reference for the innovation of lead-free solder manufacturing.     

Nonlinear transformations in Hilbert space

Vitalism, from a physicist's standpoint, suggests the introduction of nonlinear transformations in Hilbert space. Two such transformations are introduced and studied in some detail. They are hard to detect by conventional experiments, although they may be very important for living organisms. They can, however, give rise to nonlocal effects, and thus provide a possible physical basis for some parapsychological phenomena, in particular precognition.A recent history in which these two views are contrasted has been written by Smith.